Gypsy moth caterpillars often appear in their thousands! Photo: http://www.dufferincounty.ca
When I was a boy (back in the 1960s and early ’70s), I can recall the near paranoia about the gypsy moth in my area (Toronto, Canada).
The pest was, at that time, near the Canadian border, and we were warned it would soon invade and destroy our forests. We were told to check our cars and camping equipment when we came back from New England, where it was well established, and certainly never to transport firewood across the border.
Well, it did cross the border and then some, and continues to spread, but the forests weren’t destroyed. Yes, it causes very visible destruction, with entire trees defoliated and, in years of serious outbreaks, wide patches of forest barren of leaves in late July can be startling, but most trees recover and soon produce new leaves. Where it attacks repeatedly, year after year, it will undoubtedly affect the mix of species in the forest (its preferred host is the oak, although it will easily move to over 500 other tree species, including alder, apple, aspen, beech, birch, black gum, cherry, hawthorn, hemlock, hornbeam, larch, linden, maple, pine, sassafras and spruce), but it has certainly not been destroying entire forests as I had imagined.
And now, a new gypsy moth parasite may be bringing its numbers down to—dare I say?—sustainable levels.
I’ve only personally encountered gypsy moths twice.
Once was about 20 years ago, when I found a female moth laying a mass of eggs at the base of my crabapple tree. She was certainly easy to see, being so white against on the dark gray bark. Mother and progeny were quickly scraped off and I’ve never seen hide nor hair of a gypsy moth in my garden since.
The other was in Rosemère, Québec, in July 2019, when I was visiting a private garden. The owner took me over to a shady corner to show me a strange phenomenon; a cluster of what must have been thirty caterpillars on a tree trunk, all facing down. They were mostly immobile, although some nodded slightly. They didn’t react to our presence, even when we touched them. They were dying, attacked by a fatal fungus, Entophaga maimaiga (fatal to gypsy moths, that is).
That’s what this article is about, a “new” fungus that may will put the gypsy moth in its place. But first, a bit of history and general information.
The gypsy moth (Lymantria dispar) is native to Eurasia and northern Africa. There are several different subspecies, each with different host preferences. The one brought to eastern North America is the European gypsy moth (Lymantria dispar dispar). It’s the one with the oak fetish … and also one where the female insect doesn’t fly. But more on that later.
The European gypsy moth was accidentally introduced to the New World by French entomologist Étienne Léopold Trouvelot (1827–1895). He brought it from Europe in 1869 with the idea of crossing it with silkworms (Bombyx mori) in order to develop a silkworm industry in New England. However, the moths soon escaped from his residence in Medford, Massachusetts.
At first, they had little impact, but in 1889, therefore 20 years later, a major outbreak occurred, with entire forests being defoliated and caterpillars covering houses and sidewalks.
The gypsy moth then slowly spread across New England and is now present throughout most of the Eastern United States and southeastern Canada. It occasionally pops up in Western North America too, but such local infestations are quickly quashed by local authorities.
One thing that intrigued me as a child was that I was told the female moth of the European subspecies didn’t fly, but the males did. I just couldn’t figure it out: if female gypsy moths don’t fly and they laid the eggs of the next generation, how could gypsy moths possibly spread? Wouldn’t the females just live on the tree where they hatched or, at most, a neighboring tree, and lay their eggs there? That wouldn’t get them very far!
It turns out that, after hatching, some of the larvae, still tiny, move high up into trees, then hang by threads from branches. From there, they can be blown considerable distances, possibly using the thread as a parachute. Apparently, that’s how they crossed Lake Michigan to reach Wisconsin, a distance of at least 50 miles (80 km)!
Without intervention, gypsy moths spread on average some 13 miles (21 km) per year. Pretty good for an insect with stubby legs that can’t fly!
Gypsy Moth Life Cycle
There is only one generation per year.
In North America and Asia, adult gypsy moths appear in late July or August. They’re rather nondescript moths, hard to distinguish from other species. The brownish males with feathery antennae fly mostly at night and mate with several females. The latter are white with black markings, have thin antennae and, despite the fact they have wings, do not fly.
They give off a powerful pheromone that attracts males from afar. Adults neither feed nor drink and die after about a week, but not before the female has laid about 100 to 1,000 eggs, usually on the lower part of tree trunks or on shrubs, rocks, stumps, vehicles, outdoor furniture, sides of buildings or firewood.
If seen, the egg masses have a spongy appearance, covered with tan or buff hairs, and can be scraped off and disposed of. Wear gloves, as the hairs are irritating.
Eggs overwinter where they are laid. They can be killed by severe cold: 16˚F/-9˚C is sufficient if maintained over a long period, but -9˚F/-23˚C can kill them after only a few hours. Still, they can survive under snow and other protected spots in colder climates.
Dormant oil spray can be used to destroy the eggs.
Eggs hatch in the spring, about the time the first leaves appear on trees. The larvae look much like the caterpillars of other forest caterpillars such as tent caterpillars (Malacosoma spp.): blackish and hairy. Both types are mostly noticed not only because of the extreme damage they cause (serious defoliation), because they appear in such large numbers, something especially noticeable as they grow in size, from only 1/8 inch (3 mm) in length at first to up to about 2 1/2 inches (60 mm) just before pupation. As they reach maturity, you can more easily tell gypsy moth caterpillars from other species by the double rows of five raised blue dots near the head followed by six similar red dots.
Caterpillars can be squashed or dropped into soapy water. The hairs are irritating to the skin and can even cause allergic reactions, so wear gloves. Or spray them with Btk (Bacillus thuringiensis kurstaki), a widely available biological insecticide. Surrounding the trunk of an infested tree with a sticky product like Tanglefoot can also trap them.
The caterpillars also produce frass (caterpillar poop), often in huge quantities, covering decks and garden furniture. You can even hear it hitting the ground, like little drops of rain. Besides being annoying and disgusting, it can be quite slippery after a rain and requires a lot of cleanup. But, to give frass its due, it does feed the forest in minerals.
By mid-June to early July, the caterpillars leave their tree and look for a hiding place—between ridges of bark, under loose bark, under branches or leaves, in fissures in rocks, on the ground, etc.—and turn into pupae. This is also when they most often hitch rides on vehicles, camping furniture and firewood. The pupae are brown and shell-like, somewhat hairy. Again, if you find any pupae, they can be squashed or dropped into soapy water.
The adults emerge after 14 to 17 days … and a new cycle begins.
Predators and parasites
Since the gypsy moth was first accidentally released in North America, all sorts of parasites and predators have been imported from Eurasia and released in order to control it. Many failed to adapt and disappeared, but there are now several parasitic flies and wasps that are well established over the gypsy moth range and are reducing its numbers. And a few native parasitic wasps also help. Also, several viral diseases have become established and can cause significant mortality. One, Lymantria dispar multicapsid nuclear polyhedrosis virus (LdMNPV), often shortened to NPV, is also used as an insecticide under the name Gypchek.
Birds, too, eat gypsy moth adults, larvae and eggs. In Eurasia, they’re apparently a major source of control, but North American birds have been slow to adapt, possibly put off by the caterpillars’ irritating hairs, and are not considered effective controls. The very common white-footed mouse (Peromyscus leucopus) has proved a very good control, though, at least where moth populations are sparse, and shrews too can be effective predators.
New Kid on the Block
However, a new pathogen, specific to gypsy moths (thus not harmful to other moth species), seems to be outcompeting the others: gypsy moth fungus (Entophaga maimaiga). (Maimaga is the Japanese common name for the gypsy moth.)
Although first introduced from Japan to North America near Boston in 1910 and 1911 during an early attempt to control the gypsy moth invasion, it apparently never established and the release was considered a failure. Further releases were made in 1985 and 1986 in New York State and Virginia. Again, the releases were considered unsuccessful.
However, in 1989, the fungus re-emerged in New England. Some entomologists believe the new cases derive from the original release (1910/1911), although no one knows where the fungus spent the intervening 75-odd years. It had apparently been slowly adapting to North American conditions all that time.
Gypsy moth fungus has since been spreading rapidly throughout gypsy moth territory. It is so effective that is relegating other disease and insect pathogens to very secondary roles.
Gypsy moth fungus will certainly never eliminate the gypsy moth entirely, but some entomologists feel it may well be the pathogen that will finally keep the gypsy moth in check. Certainly, in areas where this fungus has been found, gypsy moth populations quickly crash and rarely reach more than moderate levels thereafter.
The Fungus Life Cycle
Wind-borne spores of gypsy moth fungus land on the caterpillars and infect them by penetrating their skin.
Early in the season, after overwintering on the ground, spores are carried by wind to young caterpillars up in the canopy of the tree. The tiny caterpillars die mostly sight unseen, clinging to leaves.
From these small cadavers, the fungus soon produces spores that are carried to larger caterpillars. The latter then gather in groups on tree trunks, always head down, front legs free, arching somewhat outwards. This mass display makes the fungus infestation much more visible. The caterpillars die, turn rubbery, then shrivel. Eventually, they become covered with fungal growth, then drop to the ground and decompose, but the spores released can live in the ground for up to 12 years, assuring long-term control.
If you see a cluster of dark hairy caterpillars on a trunk, possibly nodding slightly—and it’s a more common occurrence every year!—, you’re seeing the start of the demise of the scourge of the northeastern forest, the gypsy moth, soon to be relegated to the status of—everyone hopes!—a rather minor problem!